20 "LL. J. PETERS AND J. BARDEEN 
ally and vertically. Consequently, the difficulties of interpreting the measure- 
ments become more and more difficult. Up to the present time information of 
immediate value in oil geology has not been obtained below a depth of about 
2000 feet. 
In the alternating current case there is a definite depth of current penetra- 
tion for a given frequency and a given earth resistivity. In attempting to work 
at deeper and deeper depths, the operating frequency must be lowered con- 
tinually. In addition to the difficulties mentioned in the direct current case, 
one is confronted with the problem of measuring very weak electric and 
magnetic fields of low frequency. In some cases where the resistivity of the 
surface materials is high the alternating current field is less influenced by 
lateral irregularities in the surface layer than the direct current field. In the 
present state of the art, it would take exceptionally favorable conditions to 
obtain reliable information at a depth much in excess of 1500 feet. 
At the present time electrical methods of prospecting for oil seem to be in 
disrepute. This is partly due to cost of electrical surveys as compared with 
other geophysical methods and partly due to the failure of the extravagant 
claims made for the process to materialize. However, the electrical method of 
prospecting for oil cannot be forgotten because it is one of two prominent 
geophysical methods in which it is possible to control the field being em- 
ployed. Improvements in methods of interpretation and in field technique 
should give electrical methods a definite field of usefulness in prospecting for 
oil. 
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